1. Field of the Invention
This invention relates generally to a disc driver for driving a disc, and more particularly to a method for detecting a mirror signal from a disc and a circuit therefor.
2. Description of the Prior Art
In recent, a disc accessed optically, such as a compact disc(CD) and a digital versatile(or video) disc(DVD), has been widely used as a recording medium. Generally, in this disc, tracks for recording an information are provided in a concentric or spiral shape and mirror regions for totally reflecting a light beam exist between the track. The mirror regions allow the disc driver to perform an appropriate control by indicating a boundary between the tracks. Signal pits are recorded in each of the track.
Further, the disc driver has an additional function, such as a time search and a variable bit rate control, besides a basic function of recording and reproducing an information on and from the disc. In order to perform the time search and the variable bit rate control, the disc driver must frequently move (or jump) from a track being currently accessed into a track spaced by several to tens of tracks therefrom. The disc driver must detect the jumped track number such that it can be moved into a desired track at the time of track-jumping. To this end, the disc driver must detect a signal that changes as the track and the mirror region are alternated, hereinafter referred to as a mirror signal. The mirror signal has a small amplitude when a light beam is positioned on the track while it has a large amplitude when a light beam is positioned on the mirror region. This results from a light beam being partially reflected by the track while being totally reflected by the mirror region. Since such a mirror signal is detected from a radio frequency signal generated at a pickup reading tracks of a disc in the radial direction, by means of a mirror signal detecting circuit, it is influenced by a driving condition of the disc and a state of the disc. Particularly, if a defect area due to dusts or scratches or the like occurs in the disc, then the mirror signal includes an error so that it can not temporarily indicate the track and the mirror region. This is caused by a fact that the mirror signal detecting circuit responds instantaneously with respect to a noise of impulse component involved in the radio frequency signal by the defect area.
In real, as shown in FIG. 1, the conventional mirror signal detecting circuit includes a first operational amplifier 11 for receiving a radio frequency signal RFS, and a signal detection and hold part 12 and a second operational amplifier 13 which are connected to the operational amplifier 11 in serial. The radio frequency signal RFS is generated at a pickup, not shown, for converting a light signal reflected from the disc into an electrical signal, and which has a waveform as shown in FIG. 2 at the time of track-jumping. Referring now to FIG. 2, the radio frequency signal RFS has a relatively high voltage a when a light beam is positioned on the mirror region while it has a relatively low voltage b when a light beam is positioned on the track, that is, the region in which signal pits exist. In addition, in the case where a light beam is positioned on a defect area or region, the radio frequency signal RFS has a lower voltage c, hereinafter referred to as "low-voltage noise", than the voltage b when a light beam is positioned on the track. Such a radio frequency signal RFS is applied to the signal detection and hold part 12 after it is amplified with a predetermined gain by means of the first operational amplifier 11. The signal detection and hold part 12 detects an envelope signal of the amplified radio frequency signal RFS outputted from the first operational amplifier. The signal detection and hold part 12 detects the peak voltage a and the bottom voltage in the radio frequency signal RFS outputted from the first operational amplifier 11 and holds them during a certain time, respectively. As a result, the signal detection and hold part 12 generates a peak voltage signal TS and a bottom voltage signal BS. Circuits for detecting the peak voltage and the bottom voltage from the amplified radio frequency signal RFS can be configured by capacitors, diodes and current sources, etc. In these circuits, a time constant of the bottom voltage detecting circuit is established to have such a relatively short value that an error component caused by a vibration of the disc occurring at the time of a rotation of disc can be detected. The second operational amplifier 13 makes a differential amplification of the bottom envelop signal BS and the peak envelop signal TS applied to its non-inverting terminal(+) and its inverting terminal(-), respectively, from the signal detection and hold part 12, thereby generating a peak voltage signal CRF which is compensated for noises occurred by rotation of the disc. As a result, the first and second operational amplifiers 11 and 13 and the signal detection and hold part 12 serve to detect a peak waveform of the radio frequency signal RFS correcting an error amount of the radio frequency signal RFS based on a minute change in the driving condition of the disc.
Furthermore, the conventional mirror signal detecting circuit includes a smoothing part 14 for converting the corrected peak voltage signal CRF from the second operational amplifier 13 into a direct current voltage, and a comparator 15 for comparing the corrected peak voltage signal with an output signal of the smoothing part 14. The smoothing part generates an average voltage of the corrected peak voltage signal CRF. To this end, the smoothing part 14 is composed of a first and second resistors R1 and R2 that are connected in serial between a first node N1 and a ground voltage source GND, a third operational amplifier 16 and a diode D1 that are connected in a loop shape between a second node N2 positioned in the middle of the resistors R1 and R2 and a third node N3, and a capacitor C1 connected between the third node N3 and the ground voltage source GND. The first and second resistors R1 and R2 makes a voltage division of the corrected peak voltage signal CRF passing through the first node N1 from the second operational amplifier 13 and applies the voltage-divided peak voltage signal to a non-inverting terminal(+) of the third operational amplifier 16 connected with the second node N2. The third operational amplifier 16 makes a differential amplification of the voltage-divided peak voltage signal from the second node N2 and a average voltage signal IVS fed-back from the third node N3 into its inverting terminal(-). The diode D1 charges only higher voltage components than its operation voltage in the output signal of the third operational amplifier 16, via the third node N3, into the capacitor C1. The capacitor C1 is charged and discharged in accordance with a voltage signal from the diode D1, thereby allowing an average voltage signal IVS to appear on the third node N3. When a radio frequency signal RFS includes a low-voltage noise, the average voltage signal IVS increases suddenly and then decreases slowly in a region where the low-voltage noise is positioned. The comparator 15 compares the corrected peak voltage signal CRF from the first node N1 with the average voltage signal IVS from the third node N3 to generate a rectangular mirror signal MS. This mirror signal maintains a high logic when a level of the corrected peak voltage signal CRF is higher than that of the average voltage signal IVS, that is, when a light beam is positioned on the mirror region; while it maintains a low logic when a level of the corrected peak voltage signal CRF is lower than that of the average voltage signal IVS, that is, when a light beam is positioned on the track. Also, when the corrected radio frequency signal CRF includes a low-voltage noise c, as shown in FIG. 2, an error component d having a high logic state during a certain interval from a starting point of the low-voltage noise is generated in the mirror signal MS.
As described above, if a disc has a defect area due to a dust or a scratch or the like, then a low-voltage noise caused by the defect area is generated in the radio frequency signal at the time of time search or variable bit rate control so that a mirror signal derived from the radio frequency signal can not identify the mirror region and the track temporarily. Owing to this, the disc driver can not accurately recognize the number of tracks which is passed through by a light beam, that is, which is jumped, and at the same time becomes to waste a long time for positioning a light beam onto a desired track. As a result, the conventional disc driver is unable to perform the time search or the variable bit rate control accurately.